1. Field of the Invention
This invention relates to an apparatus for conveying an image recording sheet, such as a stimulable phosphor sheet. This invention particularly relates to an apparatus for accurately conveying an image recording sheet by causing the image recording sheet to be held on a front surface of a conveyor belt by magnetic attraction such that the image recording sheet may be in close contact with the front surface of the conveyor belt.
2. Description of the Prior Art
As image recording sheets, there have heretofore been known stimulable phosphor sheets as disclosed in, for example, U.S. Pat. No. 4,315,318. The stimulable phosphor sheet is provided with a layer of a stimulable phosphor capable of storing a radiation image thereon. The radiation image, which has been stored on the stimulable phosphor layer, is read out by exposing the stimulable phosphor to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, the emitted light being detected photoelectrically. Specifically, the image read-out operation is carried out by two-dimensionally scanning the stimulable phosphor sheet with a light beam serving as stimulating rays, such as a laser beam, sequentially detecting the light, which is emitted by the stimulable phosphor sheet when it is exposed to the light beam, with a photodetector, such as a photomultiplier, and thereby obtaining an image signal representing the radiation image.
In general, the two-dimensional scanning is carried out by moving the stimulable phosphor sheet in one direction, and scanning the stimulable phosphor sheet with the light beam in the direction, along which the stimulable phosphor sheet moves, and in the direction normal to the direction, along which the stimulable phosphor sheet moves. In this method, it is required for the stimulable phosphor sheet to be moved in one direction at a constant speed. Specifically, if the stimulable phosphor sheet is not moved accurately at a predetermined speed, read-out nonuniformity will occur during the operation for scanning the stimulable phosphor sheet and reading out the radiation image therefrom. As a result, when a final visible image is reproduced from an image signal obtained from the image read-out operation, a visible image having uniform image density cannot be obtained. Therefore, an accurate mechanism is required to move the stimulable phosphor sheet.
As the mechanism for moving a stimulable phosphor sheet, there has heretofore been known a mechanism, wherein the stimulable phosphor sheet is placed on a front surface of a conveyor belt, and the conveyor belt is moved at a predetermined speed. In this case, such that the stimulable phosphor sheet may not shift in position from the conveyor belt, it is important to hold the stimulable phosphor sheet on the front surface of the conveyor belt such that the image recording sheet may be in close contact with the front surface of the conveyor belt. As the method for holding the stimulable phosphor sheet on the front surface of the conveyor belt such that the image recording sheet may be in close contact with the front surface of the conveyor belt, a method has been proposed in, for example, Japanese Unexamined Patent Publication No. 59(1984)-7946. With the proposed method, a magnetic material layer is overlaid on the back surface of a stimulable phosphor sheet such that the stimulable phosphor sheet may be attracted by magnetic attraction. The stimulable phosphor sheet is held on the front surface of a conveyor belt by magnetic attraction of a magnet, which is located on the back surface side of the conveyor belt, such that the image recording sheet may be in close contact with the front surface of the conveyor belt.
However, several problems are encountered in cases where a stimulable phosphor sheet is brought into close contact with the front surface of a conveyor belt by using a magnet. For example, in cases where the magnet is located such that it may be in contact with the back surface of the conveyor belt, resistance occurs against the conveyor belt due to friction with the magnet. If the resistance is large, the speed, with which the conveyor belt moves, will fluctuate, or the magnet will get worn quickly. In order to eliminate such problems, it is necessary that the wear-resistant properties of the magnet are improved, that the coefficient of friction of the magnet is lowered, and that the conveyor belt is driven with a high-torque motor. Therefore, the cost of the mechanism for moving a stimulable phosphor sheet cannot be kept low.
It is also considered to solve the aforesaid problems by locating the magnet in spaced relation to the back surface of the conveyor belt such that the magnet may not come into contact with the back surface of the conveyor belt. In such cases, the problems occur in that, when the stimulable phosphor sheet is attracted by the magnet, the conveyor belt bends, and the height position of the surface of the conveyor belt fluctuates. If the height position of the surface of the conveyor belt fluctuates, the height position of the front surface of the stimulable phosphor sheet will also fluctuate. Fluctuations in the height position of the front surface of the stimulable phosphor sheet also cause read-out nonuniformity to occur during the operation for reading out an image from the stimulable phosphor sheet by scanning it with a laser beam.